A Hezbollah commander has been killed on "jihad duty" in Iraq, indicating the Lebanese group that is already fighting in Syria's civil war may be involved in a second front in the region.

Sources in Lebanon told the Reuters news agency on Thursday that the Hezbollah commander Ibrahim al-Hajj was killed in a battle in Tal Afar near Mosul, a city in northern Iraq seized last month by the Islamic State group.

Hezbollah has not previously announced any role in the conflict in Iraq, which escalated last month when radical Sunni militants seized large areas of territory from the Shiite-led government in Baghdad.